'tantamount' originates from Anglo-Norman, specifically the word 'tant amunter,' where 'tant' meant 'so much' and 'amunter' meant 'to amount.'

'tant amunter' transformed into the Middle English word 'tantamount,' and eventually became the modern English word 'tantamount.'

Initially, it meant 'to amount to as much as,' but over time it evolved into its current meaning of 'equivalent in seriousness to.'